Why Thunderbird has private IP address as remote host?

Why Mozilla Thunderbird configured to access Yahoo IMAP server, uses a private IP address, even if there is no other device in the network. Every time, Thunderbird is restarted, it changes remote-host IP address in the range of 10.0.0.0/8. Further, the personal firewall has no log showing communication with any private IP.

Am I victim of Man-in-the-Middle attack, caused by the Home WiFi Router?

Thunderbird About:Config